1. Field of the Invention
This invention relates to a LAN cable.
2. Description of the Related Art
As shown in FIG. 8, conventionally, a LAN cable 81 referred to as a foiled twisted pair cable (FTP) is known that is configured to include a pair or plural pairs of pair twisted wire 82, and a metal shielding member (shield) 83 and a sheath 84 formed on the peripheries of the pair twisted wire 82 in this order (for example, refer to JP-2011-198634 A1). Further, in FIG. 8, in order to simplify the drawing, only a pair of pair twisted wire 82 is shown.
The LAN cable 81 has a small outer diameter (cable outer diameter), and an alien XT in the LAN cable 81 is also low, thus a large number of cables can be installed in a cable tray, consequently, the LAN cable 81 has a merit that an installation space factor is high. However, there is a problem that a common mode noise that is received via a transmission device receiver is heightened due to an induction voltage floating-induced by the metal shielding member 83, thus a multilevel modulated signal and a demodulated signal are buried in the noise, so that necessary transmission capacity cannot be ensured. Further, the alien XT means a crosstalk that is received from the other cables adjacent thereto.
Hereinafter, the reason why the LAN cable 81 can prevent the alien XT and the common mode noise is heightened will be explained in particular.
The pair twisted wire 82 transmits a differential signal, thus reversed-phase electric currents flow in two covered wires (covered bodies) 85 constituting the pair twisted wire 82 (in FIG. 8, the electric currents are shown as a cross mark and a dot mark), consequently, a high frequency electromagnetic field (high frequency alternation electromagnetic field) is generated in the pair twisted wire 82. In case of the LAN cable 81, the high frequency electromagnetic field generated in the pair twisted wire 82 is shielded by the metal shielding member 83, thus approximately total amount of high frequency electric current flows in the surface (eddy current skin thickness part) of the metal shielding member 83 in the side of the pair twisted wire 82. Further, in case of using a member comprised of aluminum such as an aluminum foil as the metal shielding member, the skin thickness in the frequency of 100 MHz is approximately 6 μm. Consequently, the induction electromagnetic field does not reach the pair twisted wire 82 of the LAN cable 81 adjacent thereto, or even if has reached, it is extremely decayed, thus the alien XT is reduced to the extent that it does not exist. In case of the LAN cable 81, the alien XT can be reduced by the metal shielding member 83, thus it is not necessary to enlarge a distance between the pair twisted wires 82 of the LAN cables 81 that are adjacent to each other, so that the outer diameter can be reduced.
On the other hand, as mentioned above, in the LAN cable 81, the high frequency electromagnetic field generated in the pair twisted wire 82 is shielded by the metal shielding member 83, thus approximately total amount of high frequency electric current flows in the metal shielding member 83, consequently, a floating voltage induced by the metal shielding member 83 is heightened. The floating voltage is induced to a receiver as the common mode noise, so as to cause a noise that exceeds discrimination level of the multilevel modulated signal.
As shown in FIG. 9, generally, FTP generates a common mode noise extremely higher than an unshielded twisted pair cables (UTP). Accordingly, in case of the conventional LAN cable 81, a needed frequency band cannot be ensured sufficiently, thus it is difficult to ensure a transmission capacity of 10GBASE-T (10 Gb/s-E transmission using the pair twisted wire) without code error compensation.
Consequently, in order to ensure the transmission capacity of 10GBASE-T without code error compensation, it is preferred to use an unshielded LAN cable referred to as UTP that does not include the metal shielding member 83 that generates the common mode noise.
As the LAN cable referred to as UTP, a LAN cable 101 shown in FIG. 10 and a LAN cable 111 shown in FIG. 11 are generally known, the LAN cable 101 being configured such that a sheath 84 is formed so as to cover the periphery of a pair or plural pairs of a pair twisted wire 82 in a lump (namely, configured such that the metal shielding member 83 is omitted from the LAN cable 81 shown in FIG. 8), and a LAN cable 111 being configured such that projections 112 having a rectangular shape in section are further formed on the inner side of the sheath 84.